1. Field of the Invention
The present invention relates to a video signal converting apparatus for converting high definition television signals band-width compressed by offset sub-sampling to an existing standard television signal.
2. Description of the Prior Art
High definition television (HDTV) signals occupy the bandwidth above 20 MHz, and band-width compression is necessary for direct satellite broadcasting or other similar means. An effective technology for significantly compressing the HDTV signal band-width uses sub-Nyquist sampling for band-width compression, and two such methods have been proposed: MUSE (Multiple Sub-Nyquist sampling Encoding) and HD-MAC (High Definition-Multiple Analogue Component) methods. (See Y. Ninomiya, et al., An HDTV Broadcasting System Utilizing a Bandwidth Compression Technique - MUSE (IEEE Trans. Vol. BC-33, No. 4, p. 130 (1987)), and F. W. P. Vreeswijk, et al., HD-MAC Coding for Compatible Broadcasting of HDTV Signals (Symposium Record Broadcast Session 1989, pp. 37-53).
As shown in FIG. 22, these methods use inter-field and inter-frame offset sub-sampling to process four fields in one sampling phase so that each field transmits only 1/4 the total sampling points of the source signal. More specifically, according to the MUSE system, the first field in the first frame (referred to as a leading frame) carries pixel data "A" aligned in every other pixel positions in odd numbered horizontal lines; the second field in the leading frame carries pixel data "B" aligned in every other pixel positions in even numbered horizontal lines; the first field in the second frame (referred to as a trailing frame) carries pixel data "C" aligned in the remaining pixel positions in the odd numbered horizontal lines; and the second field in the trailing frame carries pixel data "D" aligned in the remaining pixel positions in the even numbered horizontal lines.
The receiver then restores the source signal by interpolating the sampling points that are not transmitted (the "unsampled points" below) from the received sampling points.
For forming stationary areas of a television picture, the pixel data sampled in the current field are interpoled with the pixel data obtained in three previous fields, as shown in FIG. 22 to form a complete image at the stationary areas.
For forming moving areas of a television picture, only the pixel data sampled in the current field are used.
Because the interpolation methods used for stationary and moving areas are different, the receiver must detect motion in the image to restore the original image by mixing signals processed as stationary images and signals processed as moving images according to the amount of detected motion.
A television receiver with a built-in decoder (signal processor) performing this interpolation process is needed to receive HDTV broadcasts, but these are currently extremely expensive. It will be some time before such televisions become common in the average household. MUSE-NTSC converters, which convert MUSE-format HDTV signals to the current standard NTSC signal are therefore available to enable display and recording of MUSE-format HDTV broadcasts using the standard format televisions and video cassette recorders (VCR) that are already common in the home. Several video signal converters for outputting the standard format TV signal after image interpolation have been proposed.
The simplest converters restore the image using intra-field interpolation whereby only the unsampled points are interpolated from the intra-field sampling points using a two-dimensional (2D) filter. This process is essentially the same as the moving image process of the MUSE decoder. The signal transmission characteristics of the intra-field interpolation circuit are shown in the frequency characteristics graph of FIG. 23a. In a stationary image, high resolution components are aliased to a low frequency band by two types of offset sub-sampling for transmission in the MUSE format as shown by the frequency characteristics graph in FIG. 23b. As a result, the aliased high resolution component is reproduced as a low frequency component and image deterioration known as "aliasing interference" occurs from interpolation by an intra-field interpolation circuit with a signal pass band width as shown in FIG. 23a.
In the MUSE-NTSC converter, aliasing interference is removed by time-base inter-field and inter-frame processing to achieve high image quality.
One video signal converter offering high image quality is described in Development of a MUSE-NTSC Converter, The Institute of Television Engineers of Japan Annual Conference 1991 (ITEC '91, 14-9).
In a conventional video signal converter as shown in FIG. 24, there are two interpolation circuits, for stationary and moving areas, as in the MUSE decoder, and images are restored by mixing the image areas processed as stationary and moving areas according to the detected movement in the image.
In this converter, the moving image interpolation circuit uses intra-field interpolation. The stationary image interpolation circuit, however, applies inter-frame interpolation to the input MUSE signal to substitute half of the pixels in the current field into the signal delayed one frame period, and uses inter-field averaging thereafter. Because of this inter-frame interpolation process, the high resolution component aliased as shown in FIG. 23b becomes the aliased component resulting from inter-field offset sub-sampling as shown in FIG. 13. This component is then removed by the inter-field averaging process. Aliasing interference caused by offset sub-sampling is thus removed in the resulting signal.
The outputs of two interpolation circuits are therefore selectively used according to the amount of movement detected in the image by the movement detection circuit. The restored image is free of aliasing interference in the stationary image areas, and such image deterioration as double images (ghosts) do not occur in the moving images. The number of scan lines and other parameters are then adjusted to complete the conversion to a standard television signal.
The problem with this technology, however, is that the same amount of memory used by a MUSE decoder is required for stationary image processing at the HDTV signal rate, and the converter is therefore expensive.
In addition, vertical resolution deteriorates because field aliasing interference is removed by the field averaging process.